With increasing development of technology, computers become essentials of our lives. As common electrical appliances, reliable and stable power is necessary for activating the computers. As known, a power supply apparatus is widely employed to convert an alternating current (AC) from a regular plug into a direct current (DC) to be used by the computer. For a purpose of maintaining desirable performance of the computer, the power supply apparatus should meet with specified requirements and specifications associated with safety, reliability, protection, EMC (electromagnetic compatibility), etc.
Referring to FIG. 1(a), a functional block diagram of a conventional power supply apparatus is shown. The power supply apparatus comprises a rectifier 11 and a DC-to-DC converter 12. An input AC voltage Vin received by the rectifier 11 is firstly rectified to a high DC voltage V, which is then converted by the DC-to-DC converter 12 into a low DC voltage Vout. The low DC voltage Vout is outputted to be used by a load 13 such as an electrical appliance.
FIG. 1(b) is a schematic circuit diagram of the DC-to-DC converter in FIG. 1(a). The DC-to-DC converter 12 is a half-bridge converter, which comprises a control chip 121, a current transformer (CT) 122, a transformer 123, a rectifier 124, a filter 125, switching transistors Q1 and Q2, and a capacitor 126.
It is found that the secondary winding of the transformer 123 induces the voltage when a current change in the primary winding of the transformer 123 takes place. In addition, the induced voltage will be further processed by the rectifier 124 and the filter 125 so as to provide the low DC voltage Vout in proportion to the turn ratio of the transformer 123 to the load 13. By controlling the discharging action of the capacitor 126 at the primary winding of the transformer 123, the purpose of inducing the secondary winding of the transformer 123 will be achieved accordingly.
Generally, the control chip 121 is utilized to control the switching statuses of the transistors Q1 and Q2. For example, when the transistor Q1 is conducted but the transistor Q2 is shut, the high DC voltage V will charge the capacitor 126 via the transistor Q1, the current transformer 122 and the primary winding of the transformer 123 sequentially. As shown in FIG. 1(c), when the current flowing through the capacitor 126 reaches the peak value I1, the control chip 121 will turn off the transistor Q1 but turn on the transistor Q2, and then, the capacitor 126 is discharged via the primary winding of the transformer 123 and the transistor Q2 until the current flowing through the capacitor 126 reaches the peak value I2. Again, the transistor Q1 is conducted but the transistor Q2 is shut to charge the capacitor 126, and successively the transistor Q1 is shut but the transistor Q2 is conducted to discharge the capacitor 126. The charging/discharging procedures are continuously performed, thereby resulting in current change at the primary winding of the transformer 123. Accordingly, the low DC voltage Vout is produced from the secondary winding of the transformer 123.
As known from the above description, the control chip 121 should dynamically detect the current flowing through the capacitor 126, and compare the detected current with a predetermined reference value. According to the comparing result, the switching statuses of the transistors Q1 and Q2 are dynamically controlled. Referring again to FIG. 1(b), the current flowing through the capacitor 126 is dynamically detected by the current transformer 122, which is electrically connected to the transistors Q1 and Q2, the control chip 121 and the primary winding of the transformer 123. The detected current signal is transmitted to the control chip 121 for comparison.
Please refer to FIG. 1(d). Another conventional DC-to-DC converter applied to the circuit of FIG. 1(a) is illustrated. The DC-to-DC converter of FIG. 1(d) also comprises a control chip 121, a transformer 123, a rectifier 124, a filter 125, switching transistors Q1 and Q2, and a capacitor 126. The main difference of the circuit in FIG. 1(d) is that a resistor R is connected to the capacitor 126 in series. The resistor R is also electrically connected to the control chip 121 in order to measure a voltage drop across the resistor R, i.e. Vc. The current flowing through the capacitor 126 can be deduced from the equation Ic=Vc/R. This detected current signal Ic is transmitted to the control chip 121 for comparison. The operation principles of the control chip 121, the transformer 123, the rectifier 124, the filter 125, the switching transistors Q1 and Q2, and the capacitor 126 included therein are similar to those shown in FIG. 1(b), and are not redundantly described herein.
The above-mentioned DC-to-DC converters have some drawbacks. For example, the DC-to-DC converter of FIG. 1(b) is not cost-effective due to provision of the current transformer 122, which is disadvantageous for competition in the market. Although the DC-to-DC converter of FIG. 1(d) is not expensive, a large energy loss is resulted from the resistor R, which is power-consuming.
Therefore, it is needed to provide a current detecting circuit that can solve the drawbacks in the prior art.